A work vehicle such as a wheel excavator normally includes a service brake mounted thereat to apply a braking force as a brake valve is driven in response to an operation of a brake pedal. This brake can be utilized as a work brake when the work vehicle is engaged in operation and, in such a case, the service brake is continuously engaged in operation by operating a brake switch. Namely, the brake is hydraulically locked. The brake device disclosed in the following priority art literature is an example of this type of brake device.    Patent reference literature 1: Japanese Laid Open Patent Publication No. H9-184169